MM-52438 Clean up web app CI and only run when web app code is changed (#23232)

* MM-52438 Only run web app CI for PRs with web app changes

* MM-52438 Don't wait to run web app tests in CI

* MM-52438 Remove commented out caching

* MM-52438 Reuse steps to set up webapp environment

* MM-52438 Add test-ci script to root package.json

* Simplify path filter

.github/actions/webapp-setup/action.yml

@@ -0,0 +1,18 @@
+name: "Web app setup"
+description: "Set up NPM and dependencies"
+
+runs:
+  using: "composite"
+  steps:
+    - name: ci/setup-node
+      uses: actions/setup-node@64ed1c7eab4cce3362f8c340dee64e5eaeef8f7c # v3.6.0
+      id: setup_node
+      with:
+        node-version-file: ".nvmrc"
+        cache: npm
+        cache-dependency-path: 'webapp/package-lock.json'
+    - name: ci/get-node-modules
+      shell: bash
+      working-directory: webapp
+      run: |
+        make node_modules